The telecommunications industry continually attempts to improve the transmitter circuitry in wireless communication systems. Power amplifier (PA) circuitry is a major component of a transmitter of a wireless communication device. Power amplifier (PA) circuitry provides the power for transmitting a signal (including data modulated and carried by the signal) so that a base station or a receiver can receive the signal.
Power amplifier (PA) circuitry uses a large amount of power. The power amplifier (PA) module is one of the most power consuming components of a wireless communication device. Therefore it is very desirable to provide power amplifier (PA) circuitry that is power efficient.
One method for improving power amplifier (PA) efficiency is to use a drain/collector modulation technique. In the drain/collector modulation technique a non-linear high efficiency power amplifier can be used (e.g., a class C power amplifier) instead of a linear low efficiency power amplifier (e.g., a class A amplifier). The power control of the power amplifier (PA) circuitry is achieved by adjusting the power amplifier (PA) power supply VCC. A high efficiency power supply combined with a high efficiency power amplifier (PA) (with constant bias) would be ideal.
In prior art power amplifier (PA) modules in GSM (Global System for Mobile Communications) telecommunication devices such as RF3110 (manufactured by RFMD) and TQM7M4014 (manufactured by Triquint), the power amplifier (PA) power supply VCC is from a linear regulator or “low drop out” (LDO) circuit. An LDO circuit can have a high efficiency when the value of its output voltage (VCC) is near the value of its input voltage (VBATT). But an LDO circuit will have a very low efficiency when its output voltage (VCC) is very low compared with its input voltage (VBATT).
The maximum efficiency for an LDO circuit is the ratio of the output voltage VCC to the input voltage VBATT. That is, the maximum efficiency is given by the ratio VCC/VBATT. For example, the maximum efficiency for an LDO in a typical GSM handset with an output voltage of nine tenths volts (VCC=0.9 volts) and an input voltage of three and six tenths volts (VBATT=3.6 volts) is twenty five percent (25%).
FIG. 1 illustrates a schematic diagram of a first prior art power supply control circuit 100. Power supply control circuit 100 comprises a low drop out (LDO) circuit 110. Low drop out (LDO) circuit 110 comprises an operational amplifier 120 that receives a VRAMP signal on its inverting input. A feedback voltage signal VFB is provided to the non-inverting input of operational amplifier 120. The operating voltage for low drop out (LDO) circuit 110 is provided by a voltage source VBATT.
The output of operational amplifier 120 is provided to a gate of a PMOS transistor 140. The source of PMOS transistor 140 is coupled to the operating voltage VBATT. The drain of PMOS transistor 140 is coupled to a first end of a first resistor 150. The second end of first resistor 150 is coupled to a first end of a second resistor 160. The second end of second resistor 160 is coupled to ground. The feedback voltage signal VFB is obtained from a node between the first resistor 150 and the second resistor 160.
The output of low drop out (LDO) circuit 110 is the power supply voltage VCC. A capacitor 170 is coupled between the output of the low drop out (LDO) circuit and ground. The power supply voltage VCC is provided to radio frequency (RF) power amplifier (PA) 130. Radio frequency (RF) power amplifier (PA) 130 amplifies an RF input signal (RFIN) to generate an amplified RF output signal (RFOUT).
One method for increasing the efficiency of the power amplifier (PA) power supply VCC is to use a switching regulator. A switching regulator is able to adjust the value of the operating voltage (designated VSWITCHER) that is provided to a low drop out circuit. FIG. 2 illustrates a schematic diagram of a second prior art power supply control circuit 200 that comprises a switching regulator 210 (designated “switcher 210”). Switcher 210 has a first input that receives a peak value of voltage (designated VPEAK) and a second input that receives an enable signal (designated EN).
The low drop out circuit 110 in FIG. 2 has the same structure as the low drop out circuit 110 shown in FIG. 1. However, the operation of the low drop out circuit 110 in FIG. 2 no longer has a single value of operating voltage VBATT. Instead, switcher 210 provides a wide dynamic range of operating voltages VSWITCHER to the low drop out circuit 110. For example, the value of the operating voltage VSWITCHER may be chosen in a range from about four hundred millivolts (400 mV) to about four and eight tenths volts (4.8 V).
There are some problems, however, that are associated with prior art power supply control circuits of the type that operate with a switcher 210. For example, assume that the power supply for the LDO control amplifier 120 is provided from the operating voltage VBATT (as shown in FIG. 2).
First, when the value of the operating voltage VSWITCHER for the LDO PMOS transistor 140 is greater than the sum of the operating voltage VBATT and the threshold voltage VTP of the LDO PMOS transistor 140, then the LDO PMOS transistor 140 will be in an “on” condition all of the time. The LDO circuit 110 will be out of control in this case.
Second, when the value of the operating voltage VSWITCHER for the LDO PMOS transistor 140 is less than the threshold voltage VTP of the LDO PMOS transistor 140, then the LDO PMOS transistor 140 will be in an “off” condition all of the time. The LDO circuit 110 will also be out of control in this case.
Therefore, there is a need in the art for a system and method that is capable of providing an improved architecture for a power supply control circuit and a low drop out (LDO) circuit that avoids these deficiencies of the prior art circuitry.